1. Field of the Invention
The present invention relates to a protection circuit for a semiconductor integrated circuit equipped with a semiconductor non-volatile memory device, more specifically, to a protection circuit provided on a semiconductor integrated circuit to protect an internal circuit from static electricity intruding at a terminal to which a program voltage for writing into the memory device is applied.
2. Description of the Related Art
In a semiconductor integrated circuit equipped with a semiconductor non-volatile memory device (hereinafter, referred to as xe2x80x9cmemory devicexe2x80x9d), information is written into the memory device by a voltage higher than a power supply voltage. Especially, a memory device of a junction breakdown-type or a fuse element breakdown-type requiring a current for writing is supplied with a program voltage for writing from the outside, and a terminal (hereinafter, referred to as xe2x80x9cprogram voltage terminalxe2x80x9d) for supplying the program voltage is provided.
Due to the existence of such a terminal, there is inevitably a possibility that static electricity intrudes at the terminal, and thus a protection circuit for protecting an internal circuit from the static electricity is provided.
The protection circuit, if composed of a diode or a transistor generally used as a typical circuit element, can not discriminate between static electricity and a program voltage which intrude at the program voltage terminal, resulting in operation even when the program voltage is supplied.
Further, the protection circuit is provided also in the case of the program voltage terminal being an input/output terminal of the internal circuit. In that case, the protection circuit is used in the state in which its input/output terminal is not supplied with a voltage higher than the power supply voltage from a power supply voltage terminal, and when a voltage higher than the power supply voltage is applied, the protection circuit operates accepting all the cases as static electricity mixing.
For example, it is assumed that the protection circuit is composed of a positive voltage protection diode for protecting the internal circuit from a static electricity of positive voltage and a negative voltage protection diode for protecting the internal circuit from a static electricity of negative voltage. It is conceivable that either static electricity of positive voltage or static electricity of negative voltage intrudes at the input/output terminal. When the static electricity of positive voltage intrudes therein, the protection circuit operates to pass the electricity to a GND terminal due to the forward characteristic of the positive voltage protection diode, and when the electricity of negative voltage intrudes therein, the protection circuit operates to pass the electricity to the power supply voltage terminal due to the forward characteristic of the negative voltage protection diode.
The configuration and operation of the conventional protection circuit as described above will be explained concretely here.
FIG. 9 is a circuit diagram showing the configuration of a semiconductor integrated circuit provided with the conventional protection circuit. This protection circuit is a circuit for protecting an internal circuit 9 from static electricity which intrudes at a program voltage terminal 4 and comprises a first circuit portion 1 and a second circuit portion 11.
The first circuit portion 1 comprises a positive voltage protection diode 14 connected between a GND line 8 and a program voltage line 10 in a forward direction as viewed from the program voltage terminal 4, and a negative voltage protection diode 16 connected between the program voltage line 10 and a power supply voltage line 12 in a backward direction as viewed from the program voltage terminal 4.
The second circuit portion 11 comprises a negative voltage protection transistor 18 connected between the GND line 8 and the program voltage line 10 in parallel with the positive voltage protection diode 14, and a positive voltage protection transistor 20 connected between the program voltage line 10 and the power supply voltage line 12 in parallel with the negative voltage protection diode 16.
A GND terminal 2 at the ground potential and an input/output terminal 3 of the internal circuit 9 are connected to the GND line 8. The program voltage terminal 4 which is a pad for supplying a program voltage and a memory device 50 are connected to the program voltage line 10. A power supply voltage terminal 6 for supplying a power supply voltage and an input/output terminal 7 of the internal circuit 9 are connected to the power supply voltage line 12.
The positive voltage protection diode 14 and the positive voltage protection transistor 20 are connected each other to protect the internal circuit 9 from static electricity of positive voltage which intrudes at the program voltage terminal 4. Further, the negative voltage protection diode 16 and the negative voltage protection transistor 18 are connected each other to protect the internal circuit 9 from static electricity of negative voltage which intrudes at the program voltage terminal 4. The memory device 50 is a memory device of a junction breakdown-type which requires a current for writing or a fuse element breakdown-type which is a type of being fused by a large current and connected between the GND line 8 and the program voltage line 10.
Meanwhile, a transistor has a PN junction as in a diode, and thus the transistor can be regarded as one diode. However, because of the existence of a gate terminal, the transistor (FET) is low in withstand voltage of the reverse junction to the PN junction as compared with the diode due to its configuration. The withstand voltage can further decrease in accordance with the voltage applied to the gate terminal.
More specifically, assuming that there is no second circuit portion 11 composed of the negative voltage protection circuit 18 and the positive voltage protection transistor 20 and thus the protection circuit composed of only the first circuit portion 1 as shown in FIG. 10, the transistor of the internal circuit 9 is often broken first. Assuming such a case, it is necessary to provide the second circuit portion 11 as in the protection circuit shown in FIG. 9.
Regardless of whether or not each of these protection circuits has the second circuit portion 11, it operates normally if the magnitude of the voltage applied to the program voltage terminal 4 is that of either the power supply voltage or the ground voltage.
However, a protection circuit comprising a typical diode or transistor as in the above-described conventional protection circuit can not discriminate between the static electricity and the program voltage which intrude at the program voltage terminal. Therefore, there is a problem that either the positive voltage protection diode 14 or the negative voltage protection diode 16 operates even when the program voltage greater than the power supply voltage is applied to the program voltage terminal 4, making it impossible to write into the memory device 50.
Further, in the case in which both of the power supply voltage and the program voltage are at negative voltage in the protection circuit shown in FIG. 10, when the program voltage is supplied to the program voltage terminal 4, a drawn current 53 flows in from the power supply voltage terminal 6 because the magnitude of the program voltage is greater than that of the power supply voltage. For this reason, there is also a problem that a current required for writing into the memory device 50 can not be obtained and, more than that, the internal circuit 9 malfunctions due to the change in power supply voltage.
If the protection circuit is configured to cope with a program voltage which can be applied thereto and operate only when static electricity with a different polarity (for example, positive voltage) from that of the program voltage intrudes at the program voltage terminal 4, the internal circuit 9 is protected from the static electricity (positive voltage), with the result that the protection circuit can operate effectively to either positive or negative static electricity. For a static electricity with a polarity (negative voltage) from which the internal circuit can not be protected, the power supply voltage is set at the same as the program voltage not to allow the protection circuit to operate.
This, however, results in an application of a voltage greater than a rated voltage from the power supply voltage terminal 6 to the internal circuit 9, and thus it is predictable that the above configuration brings about a undesirable situation for operation of a circuit element provided in the internal circuit 9, leading to a problem that the circuit element may be broken.
In consideration of the problems of the protection circuit for the semiconductor integrated circuit as described above, an object of the present invention is to resolve the problems, that is, an object is to make it possible to write normally when the program voltage is applied to the program voltage terminal of the semiconductor integrated circuit and to operate normally even when either the positive or negative static electricity intrudes therein to protect the internal circuit.
For achieving the above object, this invention is a protection circuit, for a semiconductor integrated circuit, connected between a GND line connected to a GND terminal of the semiconductor integrated circuit and a program voltage line connected to a program voltage terminal for supplying a program voltage for writing into a memory, including: a first circuit portion for passing a current generated by static electricity until the program voltage is applied to the program voltage terminal, and for breaking a connection between the GND line and the program voltage line when the program voltage is applied; a semiconductor device for passing a current to the first circuit portion for breaking the connection between the GND line and the program voltage line; and a second circuit portion for applying a voltage to the semiconductor device so that the semiconductor device passes the current to the first circuit portion for breaking the connection between the GND line and the program voltage line.
In the above-described protection circuit for a semiconductor integrated circuit, the first circuit portion is preferably constituted by connecting a resistor for suppressing the current generated by static electricity and a fuse element in series between the GND line and the program voltage line.
Further, the semiconductor device is preferably composed of an N-type MOS transistor for passing a current at a second breakdown to fuse the fuse element of the first circuit portion.
Furthermore, the second circuit portion is preferably configured to supply a gate of the N-type MOS transistor with a gate voltage for the N-type MOS transistor to pass the current at the second breakdown.
Moreover, the second circuit portion is preferably constituted by connecting a P-type MOS transistor and an N-type MOS transistor in series between the GND line and the program voltage line, so that a voltage obtained by dividing a voltage between the GND line and the program voltage line by the P-type MOS transistor and the N-type MOS transistor is taken as the gate voltage.
Further, in this invention, the fuse element is preferably configured such that top of a polycrystalline silicon film is covered with an insulating film having an opening portion for exposing part of the polycrystalline silicon film, a metal wiring film is formed from the top of the insulating film to the inside of the opening portion, and the metal wiring film formed on an inner wall face of the opening portion becomes a current concentration portion with a high resistance value.
In this case, the opening portion of the insulating film is preferably formed divided into two sides, a side for inputting current thereinto and a side for outputting current, and the number of opening portions on the inputting side is two or more.
Further, the fuse element may be composed of a metal wiring film provided with a fusion area which is thin and has a high resistance value, or composed of a polycrystalline silicon wiring film having a step portion, and a current concentration portion easy to fuse is formed in the step portion.
The above and other objects, features and advantages of the invention will be apparent from the following detailed description which is to be read in conjunction with the accompanying drawings.